Tunnel Selection -The diameter of the tunnel will be determined by the model of bow thruster and its propeller diameter. Tunnel diameter refers to the internal diameter of the tube.

The optimal tunnel length is between 2 and 4 times the tunnel diameter (measuring on the short [lower] side of the installed tunnel).
Tunnels which are too short allow the propeller to cavitate — which decreases performance and makes a lot of noise.
Tunnels which are too long have higher internal friction which reduces thrust. This becomes very noticable when the tunnel length exceeds 6 or 7 times the diameter of the tunnel.

Tunnel Installation Tips -
The tunnel should be placed as deep as possible so the propeller does not draw air from the surface - which will completely eliminate the thrust. Additionally, the deeper the prop, the greater the water pressure on the blades - which maximizes the efficiency of the propeller.
Generally the top of the tunnel should be a minimum of 1/2 x the tunnel diameter below the waterline. This is an absolute minimum and we recommend that it be at least 3/4 x tunnel diameter below the waterline. An even better distance would be 1 x tunnel diameter below the waterline.
Once you get the top of the tunnel 1 foot (30-35 cm) below the surface, other factors should be considered more important - for example, moving the thruster further forward to get better boat turning leverage.

Click here for an excellent, illustrated guide for selecting and installing bow thruster tunnels. This guide also includes many worthwhile installation tips for optimizing your (expensive) bow thruster's performance.